TOKYO—Japanese lawmakers said Tuesday they were preparing for possible parliamentary elections as soon as next month, an accelerated schedule that could give Prime Minister Shinzo Abe a chance to firm up popular support for his economic policies.

Mr. Abe is set to make a final decision by December on whether to raise Japan’s national sales tax to 10% next year from 8% currently. It would be the second recent tax increase after one that took effect in April.